I haven’t analyzed the exact behavior of the Recents tab but I assume that it shows “recently accessed” and maybe even “recently created/modified” files. E.g., if you create a vault in iCloud Drive, Cryptomator creates certain files that represent an encrypted vault. This includes a “masterkey.cryptomator” file amongst others. But you don’t have to worry about them, see:
You won’t see decrypted files in the Recents tab though, see: Changed Files missing in Recents Tab of iOS Files App · Issue #281 · cryptomator/ios · GitHub
We have deactivated this feature for security reasons. But we’re currently looking into it to enable it as an option.